The present invention relates generally to photodetection devices and more particularly to photodetecting arrays, such as photodiode imaging devices.
Imaging devices such as scanners, photocopiers, or radiation imagers can use a sensor sheet comprising a two dimensional layer of photodiodes which senses an image directed onto it. Each photodiode is connected to a transistor. When light strikes a particular photodiode, an electrical charge is generated by the photodiode. The transistor, which is coupled to that photodiode, switches the charge generated by the photodiode to other components such as an amplifier which amplifies the signal.
Conventionally, an active matrix array is a grid of pixels on a substrate with an active switching device, such a thin film transistor in the array. An active matrix imaging array of this type typically has a detecting cell which forms the pixel, where the detecting cell includes the thin film transistor and a photodiode on the same substrate surface.
FIG. 1 shows a cross sectional view of a photodetecting device of the prior art. This figure is taken from U.S. Pat. No. 5,619,033, which is hereby incorporated herein by reference. FIG. 1 shows a photodetecting device 92, which includes a thin film field effect transistor (TFT) 35. The TFT 35 comprises a metal gate electrode 1 formed on a substrate 5. The metal typically comprises refractory metals such as a titanium tungsten (TiW) layer which covers an aluminum layer. Other refractory metals such as chromium, molybdenum, or tantalum are suitable alternatives. A gate dielectric layer 10 of silicon nitride is formed over the gate electrode and the substrate 5. A layer of hydrogenated amorphous silicon 15 is formed over the gate dielectric layer 10. An etchstop 30 is formed from a layer of silicon nitride over the hydrogenated amorphous silicon layer 15. An n+ layer 20 is formed over the layer 15 and partially over the etchstop layer 30 as shown in FIG. 1. A titanium tungsten metal layer 25 is formed over the n+ layer 20 and an aluminum layer 26 is formed over the titanium tungsten layer 25. The titanium tungsten metal layer 25 serves as a barrier layer preventing the aluminum layer 26 from interacting with the n+ layer 20. Other suitable refractory metals besides titanium tungsten may also be used. The n+ layer 20, the titanium tungsten metal layer 25 and the aluminum layer 26 on the left side of the etchstop 30 form the source electrode 37 of the thin film transistor 35, and the n+ layer 20, the titanium tungsten metal layer 25 and the aluminum layer 26 on the right side of the etchstop 30 form the drain electrode 38 of the thin film transistor 35.
A photodiode structure is then formed over the thin film transistor 35 as shown in FIG. 1. This photodiode is segmented so that each detecting cell has its own photodiode as describe in U.S. Pat. No. 5,619,033. Further details regarding the formation and structure of the detecting cells shown in FIG. 1 can be found in that US patent.
During operation of the array, the thin film transistor 35 is turned off to allow the photodiode 99 to accumulate charge based upon incident electro magnetic radiation, such as visible light or x-rays. This accumulated charge is a received image signal. When a control signal is received from an external controller (not shown) the thin film transistor 35 turns on and transfers the accumulated charge of the photodiode 99 to the other components (not shown) that amplify and process the received image signal.
The photodiode 99 is biased by applying a voltage to the bias contact 90. This bias voltage reverse biases the photodiode 99 during normal operation of the device. The bias voltage induces an electric field in the amorphous hydrogenated silicon layer 60. When light enters this layer 60, electron-hole pairs are created. The electrons and holes are swept by the electric field to opposite sides of the photodiode 99 and accumulate near the photodetector electrode contacts which are the conductive layer 70 and the n+ doped layer 55. When the thin film transistor 35 is turned on, the accumulated charges are allowed to flow as current through the source electrode 37 to other components such as the amplifiers which amplify the detected signal.
FIG. 2 shows an electrical schematic of a conventional photodetecting array which in this figure shows four detecting cells 105A, 105B, 105C, and 105D in the array 101. The array includes two gate lines 102A and 102B and two data lines 103A and 103B. The array also includes two bias lines 104A and 104B which are typically coupled to receive the same bias voltage which is typically a constant negative DC voltage in order to reverse bias the photodiode in each detecting cell. This reverse biasing prevents the photodiode from leaking in the dark as is well known in the art. The electrical schematic of FIG. 2 also represents the layout of an array in the prior art in which the bias lines are parallel and often adjacent to the data lines as shown in FIG. 2. Each detecting cell includes a thin film field effect transistor, such as field effect field transistors 107A, or 107B, or 107C, or 107D. Further, each detecting cell includes a photodiode such as the photodiode 109A, or 109B, or 109C, or 109D. The source electrode of each field effect transistor is coupled to a respective data line and the drain of each field transistor is coupled to the N electrode, of its corresponding photodiode. The p electrode of each photodiode is coupled to its corresponding bias line as shown in FIG. 2. The gate electrode of each field effect transistor is coupled to its corresponding gate line.
One problem with a prior art array such as that shown in FIG. 2 is the fact that the bias line may capacitively couple with its corresponding data line, thereby possibly causing errors.